A growing list of human diseases that result in chromosomal aneuploidy also result in mental and neurological disruption of brain function. Examples of such diseases include the well known Down's Syndrome which is trisomy 21 and is associated with mental retardation, and multiple variegated aneuploidy (MVA) that shows a range of aneuploidies and is associated with microcephaly and mental retardation. Recent studies from our lab have identified neuroprogenitor cells and neurons that are aneuploid in normal brain: these cells have gained or lost entire chromosomes, and are thus aneuploid. What is the function of these normally aneuploid cells, and might they be related to phenotypes observed in systemic aneuploidies? In this proposal we will begin to address the function of normal neural aneuploidy by testing the hypothesis that normal alterations in chromosomal number can alter neural cell fate and survival. Two specific aims will be pursued over the two years of this proposal. Aim 1 will determine the distribution and identity of aneuploid cells in the developing neuraxis and adult brain of mouse and adult human. It will also address the sub-hypothesis that aneuploid cells may involve non-random production or be non-randomly distributed. Spectral Karyotyping (SKY) and single-locus fluorescence in situ hybridization (FISH) will allow assessment of total ploidy within interphase cells. Aim 2 will determine the effects of chromosome loss - "loss of heterozygosity" or LOH - on cell fate using a model system. This aim will begin to assess functional roles for aneuploidy, with a focus on hypoploidy for a single chromosome. Live, monosomic cells can be isolated by use of transgenic mice hemizygous for a GFP marker that has been integrated into chromosomes containing a wild-type allele for a cell differentiation gene (LifR). Hemizygous cells will be cell-sorted to purify GFP-negative cells that have lost their wild-type allele via chromosome loss and assessed for significant effects on cell fate (LifR). This proposal will provide a framework for, and data towards, understanding the functional importance of neural aneuploidy in the nervous system.